1. Field of the Invention
The present invention relates to an image processing circuit and an image processing method and, more particularly, to an image processing circuit and an image processing method for use in a liquid crystal display.
2. Description of the Background Art
In recent years, liquid crystal displays have been utilized in various fields and have been utilized in televisions as well as in PC monitors. However, liquid crystal displays have low response speeds, thereby including the problem of degradation of display quality due to afterimages in cases where moving images are mainly displayed thereon as in TV applications. Therefore, overdrive processing methods have been applied to liquid crystal displays, in order to increase their response speeds. Overdrive processing is a processing method for, in cases where image data is moving images, setting the voltage applied to the liquid crystal to be higher than usual if the direction of data change from the previous frame to the current frame is positive, but setting the voltage to be lower than usual if the direction of data change from the previous frame to the current frame is negative. This method can improve the display quality of moving images.
As described above, overdrive processing is a method of applying, to the liquid crystal, a higher voltage or a lower voltage than a voltage which is usually applied thereto. Since a highest gray-scale or a lowest gray-scale is generally set for white image data and black image data, it has been impossible to apply, to the liquid crystal, voltages higher or lower than the voltage of the highest gray-scale or lowest gray-scale. Namely, conventional overdrive processing has had the problem that it can be applied to medium-gray-scale image data, but can not be applied to white and black image data.
In the case of a normally-white type liquid crystal display, the voltage to be applied to the liquid crystal for white display (hereinafter, simply referred to as a white voltage) is set to be a lowest value, while the voltage to be applied to the liquid crystal for black display (hereinafter, simply referred to as a black voltage) is set to be a highest value. On the other hand, in the case of a normally-black type liquid crystal display, the voltage to be applied to the liquid crystal for black display is set to be a lowest value, while the voltage to be applied to the liquid crystal for white display is set to be a highest value. In any of the cases of a normally-black type and a normally-white type, it has been impossible to perform overdrive processing for white and black image data.
Therefore, there has been suggested a method for compressing the gray-scale of image data in the direction of medium gray-scale, as described in Japanese Patent No. 3511592, as a method for performing overdrive processing on white and black image data. This method is a method of compressing the gray-scale of image data in the direction of medium gray-scale, in order to save gray-scale usable for overdrive processing above and below the white and black gray-scale. This enables applying a higher voltage or a lower voltage than the black voltage or white voltage after compression.
However, the method of compressing the gray-scale of image data in the direction of medium gray-scale reduces the number of displayable gray-scale and also reduces the voltage difference between the highest gray-scale and the lowest gray-scale of compressed image data. The contrast of a liquid crystal display is determined by the difference between the voltage to be applied for the highest gray-scale and the voltage to be applied for the lowest gray-scale, namely the voltage difference between the highest gray-scale and the lowest gray-scale of image data. Therefore, if the voltage difference is made larger, this will increase the contrast of the liquid crystal display, while if the voltage difference is made smaller, this will reduce the contrast of the liquid crystal display.
The method of compressing the gray-scale of image data in the direction of medium gray-scale had the problem of reduction of the contrast of the liquid crystal display, due to the reduction of the voltage difference between the highest gray-scale and the lowest gray-scale of compressed image data.